Cellulosic fibers find utility in many applications, including absorbents. Indeed, cellulosic fibers are a basic component of many absorbent articles such as diapers. The fibers form a liquid absorbent structure, a key element in an absorbent article.
Cellulosic fluff pulp, a form of cellulosic fibers, has been used for absorbent applications because the fluff pulp form provides a high void volume, or high bulk, liquid absorbent fiber structure. However, this structure tends to collapse upon wetting, which reduces the volume of liquid that can be retained in the wetted structure. Further, such collapse may inhibit transfer of liquid into unwetted portions of the cellulose fiber structure, leading to local saturation.
Whereas the ability of an absorbent article containing cellulosic fibers to initially acquire and distribute liquid (such as from an initial liquid insult) relates to the article's dry bulk and capillary structure, the ability of a wetted structure to acquire additional liquid (such as from subsequent and/or extended liquid insults) relates to the structure's wet bulk. Due to diminished acquisition and capacity properties related to loss of fiber bulk associated with liquid absorption, the potential capacity of a dry high bulk fiber structure such as cellulosic fluff pulp may not be fully realized, with the liquid holding capacity instead determined by the structure's wet bulk.
Intra-fiber crosslinked cellulose fibers and structures formed therefrom generally have enhanced wet bulk as compared to non-crosslinked fibers. The enhanced bulk is a consequence of the stiffness, twist, and curl imparted to fibers as a result of crosslinking. Accordingly, crosslinked fibers are incorporated into absorbent articles to enhance their wet bulk and liquid acquisition rate.
In addition to wet bulk and liquid acquisition rate, a material's suitability for use in absorbent articles may be characterized in terms of other performance properties, such as liquid permeability. As noted above, performance properties tend to result from different fiber characteristics such as fiber length, fiber stiffness, and so forth. However, relationships between some performance properties indicate the existence of trade-off trends for many cellulose fiber (and other) materials. For example, liquid permeability tends to decrease as capillary pressure, expressed in terms of medium absorption pressure, increases. As explained in greater detail below, this particular relationship manifests in a manner that can be mathematically approximated as a power curve function of the two properties, which is characteristic for many if not all materials used in absorbent applications, including cellulose fiber materials, synthetic fiber materials, blends, and so forth. Of these materials, the “trade-off” curve for cellulose fiber products is the highest, but successful efforts to raise this curve higher—that is, to produce materials that exhibit better liquid permeability value at a given capillary pressure value (and vice versa) than as predicted by the power curve function described by cellulose fibers—have not yet been observed.
There are a number of methods for preparing crosslinked cellulose fibers; several are summarized in U.S. Pat. No. 5,998,511 to Westland, et al. Much effort has been spent improving crosslinking processes, such as to lower production and/or material costs, to modify absorbent and/or other fiber properties of the products, and so forth. In one example, polycarboxylic acids have been used to crosslink cellulosic fibers (such as in U.S. Pat. Nos. 5,137,537, 5,183,707, and 5,190,563, all to Herron, et al., and so forth), to produce absorbent structures containing cellulosic fibers crosslinked with a C2-C9 polycarboxylic acid. Despite advantages that polycarboxylic acid crosslinking agents provide, cellulosic fibers crosslinked with low molecular weight (monomeric) polycarboxylic acids, such as citric acid, have been found to undergo reversion to a non-crosslinked condition and thus have a useful shelf-life that is relatively short. Polymeric polycarboxylic acid crosslinked fibers, however, such as disclosed in U.S. Pat. Nos. 5,998,511, 6,184,271, and 6,620,865, all to Westland, et al., amongst others, resist such aging or reversion, due in part to the participation of the polymeric polycarboxylic acid molecule in the crosslinking reaction with an increased number of reactive carboxyl groups than is the case with monomeric polycarboxylic acids such as citric acid. In another example, U.S. Pat. No. 8,722,797 to Stoyanov, et al., discloses the use of a comparatively low molecular weight polyacrylic acid having phosphorous (in the form of a phosphinate) incorporated into the polymer chain as a crosslinking agent to achieve crosslinked cellulose fibers having improved brightness and whiteness (as well as other properties) as compared to those prepared with higher molecular weight phosphinated agents or polyacrylic acid agents without phosphinates.
Thus, there is a continuing need to produce crosslinked cellulose fibers and compositions and materials including such fibers for use in absorbent articles.